Flotation of culm



Patented Nov. 8, 1938 PATENT OFFICE 2,136,074 FLOTATION or cULM' Bruce D. Crawford, Cedar Grove, N. 1.. and Harry 4 A. Grlne, Chestertown, -Md.

No Drawing. Application August 26, 1936, Serial No. 98,072

8Claims.

This invention relates to the flotation of slush culm and other refuse coal. In a more specific modification, the invention is especially concerned with the recovery of low ash coal from anthracite slush culm or other anthracite refuse.

The problem of coal refuse, both bituminous and anthracite, has long been a serious one in the industry. The refuse coal is mainly contaminated with high ash material and is notvuseable as such. As a result, serious losses of good coal in fine sizes are suffered by the industry. and they have to incur considerable expense in disposing of the slush culm, which is usually pumped into large culm banks. Many mines do not have sum- 1 cient fairly level ground for culm banks and when the banks are placed on too great a slope, rain washes the coal into the rivers and a serious nuisance frequently arises. Slush culm banks particularly bituminous coal, also tend to ignite and burn for years with serious annoyance from smoke and fumes. The losses represented by the good coal in the culm or other refuse are, of course. much more serious in the anthracite industry than in the bituminous industry, because the'unit value of anthra'cltecoal is higher than bituminous and the character of the anthracite coal results in greater degradation in the breaker and hence larger proportions of the slush culm. Flotation of coal refuse has been proposed and 3 has been used fairly extensively in Europe primarily with bituminous and semi-bituminous coal. In the United States, owing to the very low price of bituminous coal in the last few years, coal flotation has been employed sporadically only by a few companies. Efforts to float good coal out of anthracite refuse have proven to be complete failures in the past and even with bituminous coal the operations have not been successful in the United States, despite the fact that bituminous coal represents a somewhat easier flotation problem.

When refuse coal is floated, such as anthracite .slush culm, and in the following more 'speciflc discussion of the invention the description will be based on the purification of anthracite slush culm although it should be understood that the features are also applicable to bituminous coal, the culm contains a mixture of materials varying both insize and in composition. That is to say, i

reagents which are for the most part hydrocarbons such as kerosene accompanied, if necessary,

by suitable frothers, is based on wettability of the coal by the oil and the coal therefore tends to attach itself to the froth bubbles and appears in the concentrate. Since, however, the material press 15 ent in the culm grades all the way from pure coal (having only inherent ash) to a slaty gangue impregnated with small amounts of coal, it is difficult and has hitherto been impossible to obtain a sharp flotation separation with good recovery such as is commonly found in the flotation of sulfide minerals and free gold because the good coal is not entirely liberated from the gangue. The problem miaht be considered as one in which it was necessary to float a concen- 35 trate of mineral values away from a partly mineralized gangue. Due to the fact that the coal to be recovered forms a large proportion of the feed, usually somewhat over half, a very large bulk of concentrate is produced which puts a so heavy load on the flotation machines and renders the problem even more difficult.

The goal of a flotation cleaned coal from anthracite culm is less than 10% ash. It has been proposed to float slush culm by grinding down to 35 a uniform flne size. These proposals are without economic value as the grinding .plant is as large as the flotation plant and the result of grinding reduces the value of the clean tool obtained.

Previous attempts to'clean anthracite slush culm I followed the general flotation procedure which has been used with slight success on bituminous coal; that is to say, the culm with or without grinding, is floated in a string of flotation machines using cleaner machines for cleaning the 45 concentrate. There is no difllculty in obtaining a concentrate which contains a large portion of the good coal. say from up, but such a concentrate is not low in ash and cannot be economically cleaned. Attempts to improve the grade of the coal concentrate by reducing the amount of reagents used were also unsuccessful. It was possible to obtain a concentrate of the desired ash content but only at the expense of very serious losses in recovery. All attempts to vary flotation l0 procedure failed to produce both good recovery and a low ash concentrate.

The present invention is based on a type of flotation procedure which is practically unknown in ordinary flotation of minerals. It was found on careful study of the results of previous failures that concentrates which were too high in ash contained the high ash contaminants'largely in the form of "fine particles. The present invention then proceeded to eliminate the high ash material from the concentrate without losses in recovery by sizing the feed, splitting it into two or more size ranges and floating each range separately, using a different technique for the flotation of each size range. This procedure of the present invention immediately solved the problem. It was possible to obtain low ash concentrates with good recovery, (80% and over) of low ash coal in both the flner and coarser ranges, and a notable saving in reagent cost was effected because in the development of the present invention it was found that for low ash concentrate of the finer size ranges a much smaller amount of collector should be used than is necessary in the flotation of the coarser size ranges. A further saving is effected by eliminating or greatly reducing the amount of frother in floating the coarser sizes.

The concentrates of the two or more size ranges may be mixed together or sold separately. depending on the marketing economics existing.

' This is a factor in which coal is practically unique among minerals which are capable of beneficiation by flotation. Practically all other minerals are ground to a more or less uniform maximum size suflicient to liberate values and the further treatment of the concentrate, which frequently requires further grinding, is just as efiective with fine sizes as with coarse. With coal the situation is entirely different. Normally the coarser coal is worth more than the finer. .Thus it is not possible economically to reduce culm to a fairly uniminous coal.

serious problem presented by the economics of form size range before flotation and this is, of course, one of the reasons why previous efforts to float culm have been completely unsuccessful with anthracite and of doubtful success in bitu- The present invention solves the coal marketing with a saving in reagents and without additional plant costs because it has been found that fewer flotation machines are required to float a given tonnage when it is split in two or more size ranges than ifit is not so split. There is, thus, not only a saving in reagents, a higher recovery of lowerash coal, but the cost of plant and operating cost is materially reduced. A further advantage of the invention is that the flotation is less critical because the more'uniform size ranges are less subject to small temporary variations in operating procedure which may occur accidentally. All these important advantages are obtained without any material increase in cost since the only procedure required to split the feed into size ranges is a plain screening or other cheap method of classification. The present invention is, of course, inno way concerned with any particular classification method. The cheapest and most effective method for a given culm may be employed.

.While the present invention contemplates split- 70. ting the culm into two or more sizes and floating each size separately, the number of size ranges to be separately floated depends on the economic advantages obtained by further sizing. There is an enormous improvement in recovery and ash lowering when two size ranges are separately and each additional operation produces proportionately less improvement. Thus, the law of diminishing returns begins to operate very rapidly and in view of the low price of coal it is normally preferable with most culms to float two sizes, although better theoretical results can be obtained with three, it being usually cheaper to waste the small amount of coal rather than to incur the charges for further classification, small as they are. However, this is purely a question of economics and will vary with every particular refuse coal and with the marketing conditions obtaining in the area, and the present invention is therefore in no sense limited to any particular number of size ranges to be floated, so long as there are at least two ranges.

When the coal is to be divided into two sizes,

the cut may advantageously be made in the neighborhood of mesh (Tyler standard series); for

example, with anthracite culmof the St. Nicholas breaker of the Philadelphia 8; Reading Coal & Iron Company, a cut is advantageously made at 28 mesh (Tyler standard series). This permits selle ing the coarse concentrate as No. 4 buckwheat. Where the concentrates are to be mixed the cut may be made at somewhat smaller sizes. The particular point is also affected by the relative amounts of flne and coarse coal in a particular sample of culm: Y

while for many purposes it is desirable to float separately all of the size ranges into which the feed has been split, there are many culms which contain a material portion of very flne slimes which are very high in ash. These flne slimes may be minus 100 mesh (Tyler standard series) or even ilner. No important gain in good coal is obtained by including this material in the flner ranges floated and the capacity of the flotation machines is greatly reduced because of the increased flotation necessitated by the slimes. Therefore, with culms which show this type of very high ash slime, it is desirable to deslime and throw away the slimesbefore floating. the other size ranges. The invention is, of course, not limited to this feature in its broader aspects, but in a more specific modification the desliming is an advantage and is included in the invention. The procedure used in desliming may be of any standard form, that is to say, using a suitable hydraulic classifier of the types which are standard in ore dressing. The desliming of the coal prior to sizing presents no unusual classification problem.

While it is an advantage of the present invention that culm may be deslimed before sizing on any standard apparatus without modification of procedure, we have found that it is advantageous to add a small amount of an alkali metal silicate such as sodium silicate during the desliming operation. The silicate appears to accelerate the desliming and to carry over a larger proportion of high ash material. The resulting concentrates are of lower ash and the flotation is accelerated. This feature of using sodium silicate is therefore included in combination with the main features of the invention.

The present invention is useful not only in treating currently produced refuse coal but may also be used in recovering good coal from old culm banks. ,It therefore not only prevents the further waste which is involved in producing culm in current operations, but permits salvaging good coal from material which had been hitherto simply a waste. This phase of the invention is of large economic importance in the anthracite fleld although of minor importance in bituminous. The anthracite culm banks contain a very large amount of coal which can be recovered by the present invention. One large coal company alone has more than twenty million tons of culm in banks and the present invention opens up a large, new source of supply of anthracite coal. thus postponing the exhaustion of this important fuel. Coal from culm banks sometimes does not was deslimed at about 100 mesh using 1 lb. of sodium silicate per ton as a dispersing agent. The +28 mesh and the deslimed 28 mesh were then-floated in the same type Fagergren machines as described in, Example 1. The amounts of reagents for the +28 portion were the same as in Example 1 and for 28 mesh there was used 4 lbs. of kerosene per ton and .30 lb. of frother with no additional sodium silicate. The metallurgical data will be found in column 3 of Table A.

s Table .4

Float time, Float time, Composite Total reagents lbs.

cone 1 9 00110 1 mins. 2 cone 2 mins. finished cone. Rec0v ton of ori inal i Mctitor; 5:1"

WL, Ash Wt, Ash, W t. AS]! WL, A811, VIZ. Ash, theocent cent cent cent g cent cent cent cent cent cent 1 26. 81 ll. 51 23. 48 8- 14 5 3 38. 02 17. 2 26. 72 13. 6 3 I 2% 50. 2 11. 05 79. l 1. 0 7. 5 0- 72 2 21. 12. 26 19. 07 9. 04 5 4 35. 57 ll. 47 32. 10- 29 2%: 2 52- 00 9. 83 36. 5 1. 5 5. 85 0. 31 3"-" i8. 67 10. 99 16. 54 9. 26 2% 2} 35. 57 11. 47 32. 93 10. 29 2% 2 4 47 9. 94 81. 8 1. 0 5. 58 0. l5

' ing or other factors. ,The amount of reagents may therefore differ when the old culm banks are being floated. Another source of waste material is the so-called river coal which has been deposited in bars or reefs in many of the rivers of northeastern Pennsylvania.

The invention will bedescribed in greater detail in connection with a few illustrative examples. but it should be understood that it is not limited to the details therein set forth.

Example 1.

A sample of current production culm from the St. Nicholas Breaker of the Philadelphia 8: Reading Coal 8; Iron Company was treated by flotation of the full range size and by dividing into plus and minus 28 mesh sizes which were floated separately. The culm contained approximately 32.6% ash. The +28 mesh portion contained 27.35% ash and the 28 mesh contained 37.8% ash. Both flotation tests were made on standard Fagergren flotation machines. The reagents used were kerosene, sodium silicate and a frother consisting of a mixture of parafllne alcohols of 7 to 10 carbon atoms and a hydrocarbon stabilizer described in the patent of Christmann and Jayne, No. 2,065,053 issued December 22, 1936. In each case rougher concentrates were also cleaned on standard Fagergren machines. The amounts of reagents used for the flotation .of the full size range were sodium silicate 1 lb. per ton, kerosene 7.5 lbs.

- kerosene 7.25 lbs. per ton. No frother was used.

In floating the 28 mesh portion the following reagents were used: sodium silicate 2 lbs. per ton, kerosene 4 lbs. per ton, frother 0.6 lb. per ton. The metallurgical recovery from both size ranges will be shown in column 2 of Table A.

Example 2 A further portion of the same sample of anthracite culm described in Example 1 was screened at 28 mesh and the 28 mesh material Discussion of the results shown in Table A It. will be apparent from Table A that with straight flotation a slightly less than 80% recovery of coal was obtained and this coal was over 11% ash. When the coal was separated into two sizes and each size floated separately, the ash content was reduced to 9.83 and the recovery increased to 86.5. Approximately 9% increase in coal recovery was obtained with more than 10% less ash. These improved results were obtained with a material saving on kerosene, (more than 1.5 lbs. per ton) and about half the amount of the frother, (by far the most expensive reagent). Slightly more sodium silicate was used, but sodium silicate is a cheap reagent.

When the 28 mesh portion was deslimed, the grade of the concentrate remained practically the same, the recovery decreased somewhat' but there was a very marked saving in reagent, the expensive frother being cut down to slightly over one-fifth that required for a flotation of the whole feed.

In addition to obtaining better yields of a lower ash product at markedly reduced reagent consumption, the present invention resulted in a further important saving which is shown by the increased rapidity of flotation which in turn means that the cost of the flotation plant would be greatly reduced. Thus where 42 machines would be required for a given tonnage of feed, with flotation of the full size range, this number is decreased to 29 when the present invention is used without desliming and to 20 when the fine material is deslimed. Thus when using the preferred embodiment of the invention which involves both sizing and desliming, the flotation plant is less than half as great for a givenfeed tonnage. The ash content of the concentrates produced by this invention compares very favorably with the ash content of prepared domestic sizes of lump anthracite and is much lower than the fine sizes of anthracite which are frequently used for steam production.

We claim:

1. A method of cleaning slush culm which comprises classifying the culm into a plurality of sizes ranges, the out between the flnest and next size range occurring at a point substantially coarser than mesh and subjecting each size range to a froth flotation process separately with amounts of reagents and flotation time suflicient to produce a concentrate of low ash coal substantially uncontaminated with any considerable amounts of high ash material.

2. A method of-cleaning anthracite slush culm which comprises sizing the culm into a plurality of size ranges, the out between the finest and next size range occurring at a point substantially coarser than 100 mesh and subjecting each size range to a froth flotation process separately with amounts of reagent and flotation time suflicient to produce a concentrate of low ash coal substantially uncontaminated with substantial amounts of high ash material.

3. A method of cleaning anthracite slush culm which comprises desliming the culm and sizing into a plurality of size ranges; rejecting the slimes and subjecting each size range to a froth flotation process separately under flotation conditions to give a concentrate of low ash coal substantially uncontaminated by considerable amounts of high ash material.

4. A method according to claim 3 in which the desliming is efiected in the presence of an alkali metal silicate as a dispersing agent. v J

5. A method of cleaning slush culm which comprises separating the culm into two size ranges; the out between the ranges occurring at a point substantially coarser than 100 mesh and subjecting each size range to a froth flotation process, under different flotation conditions, each set of conditions being chosen to produce a concentrate of low ash coal substantially uncontaminateci by substantial amounts of high ash material.

6. A method of cleaning anthracite slush culm which comprises separating the culm into two size ranges, the out between the ranges occur-. ring at a point substantially coarser than 100 mesh and subjecting each size range to a froth flotation process under different flotation conditions, each set of conditions being chosen to produce a concentrate of low ash coal substantially uncontaminated by substantial amounts of high ash material.

BRUCE D. CRAWFORD. HARRY A. GRINE. 

